Smoke-Free Laws May Cut Preterm Births

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Active maternal smoking during pregnancy impairs fetal growth and shortens gestation. It has been suggested that secondhand smoke may also adversely affect birth outcomes. This study investigated the incidence of preterm delivery after implementation of smoke-free legislation in three phases.

There were reductions in the risk of preterm birth after the introduction of each phase of the smoking ban. No decreasing trend was evident in the years or months before the bans.

A smoking ban introduced in three phases in Flanders, the Dutch-speaking northern part of Belgium, was associated with a reduction in the risk of preterm birth, researchers found.

The rate of deliveries before 37 weeks of gestation consistently fell through successive bans on smoking in public places and most workplaces, in restaurants, and in bars serving food over a 4-year period, according to Tim Nawrot, PhD, of Hasselt University in Diepenbeek, Belgium, and colleagues.

The trends were most apparent after implementation of the last two phases, the researchers reported online in BMJ.

"Given that even a mild reduction in gestational age has been linked to adverse health outcomes in early and later life, our study has important public health implications," they wrote.

"The finding is not definitive but it supports the notion that smoking bans have public health benefits from early life," they wrote, noting that it is unlikely that sudden changes in treatments could explain the falling rates of preterm birth during the study period.

Active maternal smoking during pregnancy has been found to impair fetal growth and shorten gestation, the authors noted in their introduction, adding that secondhand smoke has also been shown to affect birth outcomes.

The three phases of the smoking ban were introduced at the beginning of January in 2006, 2007, and 2010.

The researchers collected birth data on all live-born singleton babies delivered at 24 to 44 weeks of gestation from 2002 through 2011. The analysis included 606,877 births, of which 73.9% were spontaneous. The rate of preterm delivery was 6% of the overall births and 7.2% of the spontaneous births.

From 2002 through 2006 -- before any phase of the smoking ban was implemented -- the rate of spontaneous preterm births remained stable and the rate of overall preterm births increased slightly, by 0.99% per year.

The rate of preterm birth began to drop after the first phase of the smoking ban in public places and most workplaces went into effect at the beginning of 2006, although the most apparent drops occurred after smoking was banned in restaurants in 2007 and in bars serving food in 2010.

"It might be that the implementation of nonsmoking regulation at work took place more gradually than the other bans, thus perhaps explaining why the effects of the latter were stronger (or more obvious in the analysis)," the authors wrote.

After the second phase was introduced, there was a significant reduction in the risk of spontaneous preterm birth, by 3.13% (P<0.01) and after the third phase was introduced there was a reduction of 2.65% per year (P=0.04). Similar findings were observed for overall preterm birth, as well.

According to the authors, the changes equal six fewer preterm births per 1,000 deliveries from 2007 through 2011.

The findings remained consistent after adjustment for numerous factors, including infant sex, maternal age, parity, socioeconomic status, national origin, level of urbanization, underlying trends, month of the year, day of the week, public holidays, influenza epidemics, and short-term changes in apparent temperature and particulate air pollution.

There were, however, no changes in the risks of having low birth weight or being small for gestational age when the smoking ban was being implemented.

"One could object that the effects of the smoking bans were apparent only on the rate of preterm births, and we acknowledge that the absence of an effect on birth weight and small for gestational age reduces the strength and diminishes the plausibility of our observation," the authors wrote.

They noted that other studies -- but not all -- have shown similar associations between smoking bans and preterm birth.

The researchers acknowledged that the findings may be explained by unmeasured confounders, since they lacked information on individual smoking status and other known risk factors for preterm birth, including marital status, psychosocial stressors, maternal weight, occupation, and nutrition.

The Study Center for Perinatal Epidemiology is financed and commissioned by the Flemish Center for Care and Health (Agentschap Zorg en Gezondheid). This study was supported by grants from the Flemish Scientific fund (FWO), ERC starting grant (ENVIRONAGE), and Hasselt University Fund (BOF).

The authors reported that they had no conflicts of interest.

Reviewed by Zalman S. Agus, MD Emeritus Professor, Perelman School of Medicine at the University of Pennsylvania and Dorothy Caputo, MA, BSN, RN, Nurse Planner

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